Alloy titanium has been used for all types of structures, including air craft, corrosion-resistant containers and medical implants. Titanium alloys are particularly useful for medical implants and other medical devices due to their excellent corrosion resistance and biocompatibility compared to alternative stainless steel and cobalt-chrome alloys in orthopaedic implants. Titanium and its alloys are more prone to fretting and wear compared to harder (Rc 42-44) Co--Cr--Mo alloys. The lower elastic modulus, in combination with high strength, however, allows titanium alloys to more closely approximate the stiffness of bone for use in orthopedic devices. Thus, devices formed from titanium alloys produce less bone stress shielding and consequently interfere less with bone viability. In orthodontic wire applications, a lower modulus of wire can provide a more gentle and uniform pressure over a larger displacement area than the stiffer stainless steel and cobalt alloys. Greater hardness can allow for less friction and improved motion. In cardiovascular implants, the lower stiffness of titanium alloys allows for a lower level of pressure exerted against soft tissue such as blood vessels, skin, various organ tissues and other soft tissue structures in the human body. Improved biocompatibility of titanium alloys and magnetic resonance imaging ability are additional advantages of titanium alloys.
Examples of titanium alloys used for medical devices include a low-modulus (less than 130 GPa) room temperature beta titanium alloy for orthopedic arch wire as described in U.S. Pat. No. 4,197,643. This patent describes the use of Mo, Nb, Ta and V to produce the beta alloy, and additionally, the use of Mn, Fe, Cr, Co, Ni, Cu, Al and Zr. There is no mention of the use of hafnium in the alloy. Alloy strength is achieved by aging to precipitate the alpha phase or by cold working. The preferred composition is Ti-11.5Mo-6Zi-4.5Sn, commonly called Beta III. However, the hardness is in the 30's on the Rockwell "C" scale. U.S. Pat. No. 5,312,247 describes a shape-memory or super elastic alloy having a predetermined activation temperature for use in orthopedic applications. This patent further describes the use of nickel-titanium based and titanium-molybdenum based alloys but as in the previous example, does not mention the use of hafnium (Hf) in the alloys. The use of nickel-containing metals is undesirable, not only in orthodontics, but in most medical device applications due to the common occurrence of nickel sensitivity of patients. The applicant is unaware of any Ti--Mo based alloys with shape memory properties, at least at temperatures useful in the human body. Nitinol is a commonly-used Ti--Ni alloy with shape memory behavior that is used in essentially all types of medical device applications. However, this highly elastic alloy is less than optimum with respect to other alternative available titanium alloys because the high concentrations of nickel interfere with the corrosion resistance properties of the alloys and the presence of the nickel sensitivity problem. Additionally, nickel can interfere with magnetic resonance imaging quality, and these alloys have a hardness in the Rc 30's. U.S. Pat. No. 5,232,361 describes an orthodontic bracket formulated of at least one of a group of alloys based on Ti, Zr, Si, B, Be, Cr, Nb and Co in a composition in which at least one of these elements exists in a range of between 40 weight percent and greater than 99 weight percent. An orthodontic bracket containing at least 45 weight percent titanium is given as an example. Other examples include alloys with at least 80 weight percent Ti with the addition of Al, V, Fe and/or Nb, and even a 99 weight percent Ti alloy. Once again, the use of hafnium is not described, and strength, elastic modulus, hardness, and biocompatibility are less than optimal.
Other examples of shape memory alloys include those described in U.S. Pat. Nos. 4,665,906 and 5,067,957 which describe medical devices and methods of installation using a non-specific shape memory alloy which displays stress induced martinsitic behavior, versus an activation temperature. The present inventive Ti alloy does not exhibit shape memory behavior, and contains hafnium to improve corrosion resistance and radiopacity. Additionally, unlike prior art Ti alloys described above, the presence of hafnium allows the option of surface hardening of the alloy via a conversion surface oxide, nitride, carbide, or combination of these.
Other titanium alloy device materials include those for orthopedic devices. For example, Ti-6Al-7Nb was developed several years ago to eliminate the potentially toxic effects of vanadium which is present in commonly used Ti-6Al-4V alloy (M. Semlitsch, Biomet. Technik, 1985). However, aluminum, which has been associated with Alzheimer and other neurological-related diseases, remains in this alloy. In view of this problem, others have proposed titanium alloy compositions with lower or no aluminum, or the absence of other toxic or carcinogenic elements associated with adverse responses to body function. As in the present inventive alloy, this aspect of biocompatibility has also been combined with an effort to reduce the elastic modulus of the titanium alloy, and to improve hardness over Ti-6Al-7Nb (Rc 34) and Ti-6Al-4V(Rc 34).
An early example of an improved titanium alloy for implants was discussed in U.S. Pat. No. 4,040,129 in which bone and dental implants having full tissue compatibility were described as being composed of a first component of about 3 to 30 weight percent selected from the group Nb, Ta, Cr, Mo and/or Al, and a second component of Ti and/or Zr; wherein the sum of the Cr, Mo and Al is less than 20 weight percent and the weights of Ti and/or Zr is less than 75 weight percent. This alloy was also free of Cu, Co, Ni, V and Sn. Examples described in the patent include Ti-9Nb-11Cr-3Al and Ti-4Mo-48Zr. However, the use of hafnium, surface hardenability, and radiopacity are not described.
Additionally, in U.S. Pat. No. 4,040,129, the benefit and desirability of a lower elastic modulus of the described alloy was not discussed but improved biocompatibility was. However, aluminum is found in the composition which, as mentioned above, has since been found to be associated with adverse neurological responses. No mention of the use of hafnium was given. A more recent patent, U.S. Pat. No. 4,857,269, also deals with the desirability of low elastic modulus in medical devices. This patent describes a titanium based alloy consisting of an amount of up to 24 weight percent of isomorphous beta stabilizers Mo, Ta, Nb and Zr, providing that molybdenum, when present, is at least 10 weight percent, and when present with zirconium, between 10 and 13 weight percent with the zirconium is being between 5 and 7 weight percent. Additionally, the same titanium based alloy also has up to 3 weight percent eutectoid beta stabilizers selected from Fe, Mn, Cr, Co and Ni, wherein the combined amount of isomorphous and eutectoid beta stabilizers is at least 1.2 weight percent. Optionally, up to 3 weight percent aluminum and lanthanum can be present in the alloy with the elastic modulus not exceeding 100 GPa (14.5 Msi). Examples include Ti-10-20Nb-1-4Zr-2Fe-0.5Al (TMZF).TM.. Once again, less than optimum elements (Mn, Co, Ni, Al), from a biocompatibility standpoint, are found in the alloy composition and there is no mention of hafnium or the ability to be surface hardened.
Various investigators in recent years have come to better understand the inherent biocompatibility of various elements. Laing, et al., in 1967, noted minor tissue reaction to implanted Ti, Zr, Nb, Ta and Ti alloys and a slightly greater reaction to pure, unalloyed Mo, V, Co, Ni, Mn and Fe. In another study in 1980, Steinemann concluded that vital elements Ti, Nb, Zr, Ta and Ti alloys, and Pt showed optimum biocompatibility and that the slightly less biocompatible elements included Al, Fe, Mo, Ag, Au and Co alloys and stainless steel. It was further noted that Co, Ni, Cu and V could be considered toxic. Hoars and Mears (1966) and Pourbaix (1984), based on electrochemical stability, suggested the use of Ti, Nb, Zr, and Ta as elemental constituents for improved biocompatibility. However, it is important to note that Ti--Mo alloys were included as acceptable materials and this was supported by comparative corrosion data between Ti and Ti-16Mo-3Nb-3Al in which the Ti--Mo alloy showed improved corrosion resistance. Thus, the presence of Mo in titanium alloys can actually be beneficial from the standpoint of corrosion and biocompatibility. It has also been reported in the titanium literature (Titanium Alloys, E. W. Collings, ASM, 1986) that the addition of more than about 4 weight percent molybdenum improved the corrosion resistance of titanium, particularly in crevice-type environments. With many of today's medical implants being modular in nature, this presents an issue not considered in the past. Further, to reduce fretting, corrosion and debris formation, increased hardness is desirable. The invention alloy is also designed to be greater than about 37 Rc. The above studies did not suggest the use of hafnium as a medical device metal or alloying addition. Hafnium is a sister element to zirconium and is more inert compared to zirconium.
In an effort to improve both the biocompatibility and to reduce elastic modulus in a titanium alloy, Davidson and Kovacs (U.S. Pat. No. 5,169,597) developed a medical implant titanium alloy with 10-20 weight percent Nb, or 30-50 weight percent Nb and 13-20 weight percent Zr, or sufficient Nb and/or Zr to act as a beta stabilizer by slowing transformation of beta (U.S. Pat. No. 5,545,227), where toxic elements are excluded from the alloy. The preferred example is Ti-13Nb-13Zr (Ti 1313.TM.). Tantalum can also be used as a replacement for niobium where the sum of Nb and Ta is 10-20 weight percent of the alloy. Subsequent continuation-in-part patents and applications, describing this type of alloy for cardiovascular and dental implant devices, also exist and are considered herein with respect to prior art. All of these patents and applications describe the use of Ti, Nb, and/or Zr. However, the use of hafnium, molybdenum, the combination of Hf and Mo, or small quantities of selected strengthening elements is either not described or is specifically excluded. Further, the issue of hardness is not addressed, and the preferred composition (Ti-13Nb-13Zr) has a bulk hardness of only 33 Rc and even lower (Rc 24) in the quenched condition. Others, such as I. A. Okazaki, T. Tateishi and Y. Ito, have also proposed similar compositions including Ti-15Zr-4Nb-2Ta-0.2Pd and variations of the type Ti-5Zr-8Nb-2Ta-10-15-Zr-4-8-Nb-2-4Ta, Ti-10-20Sn-4-8Nb-2Ta-0.2Pd, and Ti-10-20 Zr-4-8Nb-0.2Pd. None however, addresses hardness, the inclusion of hafnium or the ability to be surface hardened.
Teledyne Wah Chang Albany, a major supplier of titanium, zirconium, niobium and their alloys, developed a Ti-35Nb-10Zr alloy. Due to the excellent biocompatibility of hafnium (1994 Teledyne Annual Report), Teledyne also developed titanium based alloys which include niobium and hafnium and a stiff, hard, Hf-based alloy as a replacement for Co--Cr--Mo bearing alloys. However, in the Teledyne annual report, no mention was given of molybdenum and its ability to reduce elastic modulus or the use of hafnium to improve corrosion resistance, nor its ability to be surface hardened. Neither was there any mention of incorporating hafnium with molybdenum in titanium alloys. This is due, most likely, to the general perception that molybdenum has less than optimum biocompatibility. However, as mentioned previously, other studies have shown that molybdenum, combined with titanium, can have excellent corrosion resistance and biocompatibility. Further, the presence of hafnium can improve radiopacity, which is important for small implants such as clips, stents and ear replacement components, as well as for endoscopic instruments used in cardiovascular and neurological devices. Similarly important is the fact that molybdenum can reduce the elastic modulus of the alloy.
Titanium alloys have a lower hardness than, for example, Co--Cr implant alloys and stainless steels. Due to this property, many investigators have studied and reported methods to harden titanium alloys, primarily through surface hardening processes. In addition to the improved bulk hardness of the inventive alloy, the inventive alloy is also designed to be surface hardened. The improved bulk hardness further improves the attachment strength of the surface coating formed, in part, from the presence of the hafnium in the composition of the inventive alloy. Prior art surface hardening methods include a wide range of overlay coating methods such as chemical and physical vapor deposition methods. These methods, however, require too high or too low a temperature, that results in metallurgical changes and less than optimum attachment of the hard, deposited, surface coating or require the use of an interlayer to improve attachment of the hard surface coating. Oxidation and nitriding methods can form a natural conversion surface oxide or nitride with a hard, built-in oxygen or oxygen rich, hardened metal interlayer. Examples of these are described in U.S. Pat. No. 5,372,660 for zirconium-containing titanium alloys, U.S. Pat. No. 5,037,438 for oxygen surface hardening of Zr and Zr--Nb alloys for implant bearing surfaces, and U.S. Pat. No. 5,152,794 for oxidation and nitriding of zirconium or zirconium alloy trauma devices with a surface layer 1-5 microns thick. Other similar patents exist for zirconium-containing titanium alloys and Zr--Nb alloys used in orthopedic and cardiovascular devices. See, for example, U.S. Pat. Nos. 5,282,852; 5,370,694 and 5,496,359. Internal oxidation is also described in U.S. Pat. No. 5,415,704, whereas U.S. Pat. No. 5,498,302 describes internal nitridization methods to harden a surface, but without the presence of a hard outer oxide or nitride layer.
Unlike oxygen or nitrogen diffusion methods which produce interstitial strengthening of the metal, internal oxidization or nitridization, using solute levels of more oxidizable or nitridable elements in quantities less than 2 weight percent, actually forms submicron oxide or nitride dispersions to produce the hardening. Other nitridizing processes to harden the surface are described in U.S. Pat. No. 5,419,984 for stainless steel, in U.S. Pat. No. 4,511,411 for titanium alloys using an autoclave containing nitrogen, and U.S. Pat. No. 5,334,264 which uses enhanced plasma nitriding methods. There are also studies of oxygen diffusion hardening of Ti, Ti-6Al-4V and Ti-6Nb-7V alloys (Streicher), and the use of N-ion implantation (Sioshanchi) which produces a much less effectively hardened and non-uniform surface. A wide variety of surface nitriding and oxidization options are available and known to those skilled in the art. In the non-medical literature, studies by Wallwork and Jenkins, 1959, exist on the oxidation of zirconium alloys, titanium alloys and hafnium showing the oxidation of hafnium producing a hard, well attached conversion surface oxide diffusion bonded to the metal substrate. However, these oxidation characteristics were obtained in an effort to reduce (resist) this process, and not to intentionally form the surface oxide to form a hard, protective, wear-resistant surface layer. Bania and Parris (Timet, Inc., Beta 215, Vol. II, 1990 Ti Conf.) investigated various Ti--Mo, Ti--Cr, Ti--Hf, Ti--Nb alloys and other alloys with respect to oxidation resistance that leads to the optimum composition of the beta 215 alloy (Ti-15Mo-2.8Nb-3Al). Specific combinations of Ti, Mo, and Hf were not investigated for implant applications or applications with optimal combinations of strength, hardness, and elastic modulus. Although not related to medical applications, nor related to reducing elastic modulus, radiopacity, or biocompatibility, Bania and Parris have shown that the addition of 15 weight percent molybdenum reduced the oxidation resistance of Ti-15Mo-5Zr versus pure Ti. Further, the addition of 5 weight percent Hf reduced oxidation resistance to a greater degree in Ti-15Mo-5Hf. An alloy of Ti-15Cr-5Mo and Ti-15Cr also showed substantially improved oxidation resistance versus a Ti or Ti-15Mo-5Zr alloy. The best oxidation resistance in this study was exhibited by Ti-15Mo-2-5Nb, and the addition of 3 weight percent Al further improved oxidation resistance, hence the development of Ti-21S. The use of this alloy, Ti-21S, has been proposed for medical implants (Bitambri and Shetty, 1994 Soc. Biomat. Pg. 195). However, the presence of Al in Ti-21S, along with only a marginal reduction in elastic modulus in the age-hardened condition, verses the elastic modulus of Ti-6Al-4V, renders this alloy less than optimum for medical implant applications. Thus, the above discussion illustrates the non-obviousness of the inventive Ti--Mo--Hf compositions as being useful for medical implant and device applications.